Flight Risk

Three articles have been published recently examining the radiation risk posed by airport X-ray scanners. All three agree that these scanners pose at most, a very small risk to average passengers, though they disagree on the details.

When estimating the risk of cancer from a low radiation dose, scientists extrapolate downward, assuming linearity: reduce the radiation dose 100-fold and the cancer risk is also reduced 100-fold. While it has never been proven that this is true, it's the best model available.

In 2010, the Transportation Safety Authority began increased use of full body-scanners in airports across the United States. There are two types of scanner. One type uses millimeter-wave radiation, extremely low energy, non-ionizing radiation. The other type uses low-dose X-rays, which are ionizing radiation. Because ionizing radiation can cause cancer, there have been concerns expressed about the safety of the X-ray scanners.

The TSA allows passengers who do not wish to be scanned to undergo a physical pat-down.

A Risk or Are Concerns Over-Inflated?

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Two of the articles on screening appear in the April 2011 issue of Radiology. Airport Full-Body Screening: What is the Risk? was first published online by Archives of Internal Medicine on March 28, 2011 and will appear in a future print edition of the journal. The two pieces present different takes on the level of risk screening poses.

David Brenner, Ph.D, D.Sc is the director of the Center for Radiological Research at Columbia Medical Center. In his piece, Dr. Brenner says that because he only flies occasionally, he has no hesitation about going through airport X-ray scanners. But since there will potentially be up to one billion such scans performed every year, he expresses concern over the long-term consequences of these scans, particularly to frequent fliers and airline personnel. While the risks from an individual scan are small, some fliers may develop cancers from them and people who are scanned hundreds of times a year are at greatest risk. They may wish to opt for pat-downs instead.

On the low- or no-risk side is David Schauer, Sc.D, C.H.P., executive director of the National Council on Radiation Protection and Measurement. Dr. Schauer argues in his article in Radiology that the risk to an individual from airport scanning is negligible and that even critics of scanning agree with this. Schauer says that the mathematical process of taking a tiny average risk and multiplying it by large populations or time periods into a larger overall risk gives a distorted image of risk, artificially inflating a risk that an individual would accept in their daily life into something much more dangerous.

Is Very Little Risk Nothing? Or Does It Accumulate Over Time?

A major reason why one scientist can assess a particular radiation exposure as dangerous while another does not is that scientists do not know if small amounts of ionizing radiation are dangerous. One theory holds that below a certain unknown level (the threshold level), small doses of radiation pose zero risk of harm. The other theory holds that small amounts of radiation do pose a certain risk of harm, are additive over time, and that the only safe dose is zero. Both of these are theories; neither has ever been proven.

Public health officials almost all take the approach that small radiation doses carry some degree of harm. In public health, it's better to be safe than sorry.

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It would take 50 such scans to deliver the amount of radiation that a dental X-ray delivers. People receive a much higher radiation dose while in the air during an airplane flight than they do from the scan.

Many studies have been published on the risk of cancer associated with large doses of radiation, and that numerical relationship is thought to be well understood. When estimating the risk of cancer from a low radiation dose, scientists extrapolate downward, assuming linearity: reduce the radiation dose 100-fold and the cancer risk is also reduced 100-fold. While it has never been proven that this is true, it's the best model available and the one generally used to calculate cancer risk from lower radiation doses.

Schauer also points out that the risk from scanners, if any, should be balanced against the good that may come from the scanning — prevention of a security threat.

The TSA does not permit independent testing of their scanners. Schauer wants tighter regulation and testing of the scanners. He argues that radiologists are the most qualified to assess whether the scanners are operating optimally and to test their safety.

Both Brenner and Schauer agree that using millimeter-wave scanners should be a first option for airports. And they also agree that the average traveler should not be concerned about being scanned with the X-ray scanners.

Estimate: Six Cancers a Year

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Rebecca Smith-Bindman, M.D. is a professor in the department of radiology at UCSF. She has previously published a study that found that radiation exposure from medical imaging is higher than necessary and proposed ways to lower this exposure. She is co-author of a new study in the March 28 issue of Archives of Internal Medicine that concludes that the risk from airport X-ray scanners is negligible.

The amount of radiation received from a scan is the same as that received in three to nine minutes of daily living. It would take 50 such scans to deliver the amount of radiation that a dental X-ray delivers. People receive a much higher radiation dose while in the air during an airplane flight than they do from the scan.

One calculation assumed that 100 million passengers would take 750 million flights a year and that all would be scanned before each flight. For all passengers, this would result in six additional cancers developing over their lifetimes. Four of these would be to frequent fliers. At the same time, 600 cancers would develop in these frequent fliers from the increased radiation of flying at high altitude.

But she notes that it would be prudent for the TSA to permit independent testing of these scanners. Her calculations are based on the scanners operating properly and delivering a dose of 0.1 microsievert during each scan. It's not unheard of for radiation-emitting devices to malfunction. Or to be used improperly.